In a commonly-used electrostatographic reproducing apparatus, a photoconductive insulating member is typically charged to a uniform potential and exposed to a light image of an original document to be reproduced. The exposure discharges the photoconductive insulating surface in exposed or background areas and creates an electrostatic latent image on the member that corresponds to the image areas contained within the original document. The latent image is made visible by developing the image with developing material.
Most development systems employ a developer having both charged carrier particles and charged toner particles that triboelectrically adhere to the carrier particles. During development the toner particles are attracted from the carrier particles by the charge pattern of the latent image on the photoconductive insulating member to form a powder image on the member. Alternatively, single component development systems can be employed that use only toner particles. The developed image can be transferred to a support surface such as copy paper to which it can be permanently affixed by heat or pressure.
In most commercial applications the particulate developer is contained in a sump from which it is dispensed by gravity feeding. Such an arrangement inherently has a vertical dimension that is unsuitably large for a compact automatic printer. Further, functional units of the reproducing apparatus such as the imaging member, developer housing, cleaner housing, and charge corotron can be combined within a removable processing cartridge that is discarded when the developer is exhausted, the photoreceptor is worn out, or the cleaning sump is full. A desirable configuration for such a cartridge includes a generally horizontal developer sump and developer housing rather than the typical vertical, gravity feed sump and housing.
A horizontal cartridge configuration requires a mechanism to transport developer from the sump portion to the developer portion. This is particularly important in systems that require a constant supply of developer to a nip between a developer donor roll and a charge metering roll to provide an adequate quantity of charged developer to the imaging member during development.
One mechanism that has been used to transport developer is a paddle wheel that rotates through the developer and pushes it toward the rolls. However, such systems have been found to be inefficient in that they typically cannot transport all of the developer in the sump portion, so that some residual amount of developer is wasted.
Another mechanism employs a flexible sheet-like sump liner to move the developer. The liner is conformable to the longitudinal walls of the developer sump, has one end anchored to the wall near the opening in the sump through which the developer is dispensed, and has its other end attached to a roller mechanism. When the sump is full of developer, the liner conforms to the perimeter of the longitudinal walls between the roller and the anchor point. As the developer is used, the liner is taken up on the roller and pulls away from the sump walls, contracting the volume enclosed within the liner and urging the developer toward the sump opening. Such liner systems are disclosed in U.S. Pat. Nos. 4,766,457 to Barker, et al. and 4,647,180 to Watanabe. Such systems are inherently more mechanically complex than simple gravity feed systems because they require at least a liner, a roller, and some mechanism for winding the film onto the roller. Such additional parts increase the cost of the developer system and the possibility of mechanical failure and decrease the volume of the sump housing available for developer.
There is therefore a need to provide a mechanically simpler, less expensive, and lower volume mechanism to transport developer horizontally and efficiently in a developer system.